vscode-lib: provide hook to adjust configuration

This allows a client of vscode-lib to change the openctx client
configuration before it is acted upon. We are adding this hook to make
it possible for OpenCtx configuration to be set by the Sourcegraph
instance Cody is connected to.

Test Plan: TODO

client/vscode-lib/src/controller.ts

@@ -1,6 +1,7 @@
 import {
     type AuthInfo,
     type Client,
+    type ClientConfiguration,
     type ProviderMethodOptions,
     type Range,
     createClient,
@@ -56,6 +57,7 @@ export function createController({
     getAuthInfo,
     features,
     providers,
+    mergeConfiguration,
     preloadDelay,
 }: {
     secrets: Observable<vscode.SecretStorage> | vscode.SecretStorage
@@ -64,6 +66,7 @@ export function createController({
     getAuthInfo?: (secrets: vscode.SecretStorage, providerUri: string) => Promise<AuthInfo | null>
     features: { annotations?: boolean; statusBar?: boolean }
     providers?: ImportedProviderConfiguration[]
+    mergeConfiguration?: (configuration: ClientConfiguration) => Promise<ClientConfiguration>
     preloadDelay?: number
 }): {
     controller: Controller
@@ -87,8 +90,13 @@ export function createController({
     )
     disposables.push(configOrSecretsChanged)
 
+    const getConfiguration = async (scope?: vscode.ConfigurationScope) => {
+        const config = getClientConfiguration(scope)
+        return mergeConfiguration ? await mergeConfiguration(config) : config
+    }
+
     const toggleEnableCommand = vscode.commands.registerCommand('openctx.toggleEnable', async () => {
-        const currentValue = getClientConfiguration().enable
+        const currentValue = (await getConfiguration()).enable
         await vscode.workspace.getConfiguration('openctx').update('enable', !currentValue)
     })
     disposables.push(toggleEnableCommand)
@@ -103,7 +111,7 @@ export function createController({
                 ? vscode.Uri.parse(resource)
                 : vscode.workspace.workspaceFolders?.[0]?.uri
             return observeWorkspaceConfigurationChanges('openctx', scope).pipe(
-                map(() => getClientConfiguration(scope)),
+                mergeMap(() => from(getConfiguration(scope))),
             )
         },
         authInfo: getAuthInfo